On the two components of wind-driven ocean surface stress with extension to scalar fluxes
When coping with numerical models of ocean surface waves and circulation, one should differentiate between wind-driven drag due to turbulent skin friction and form drag, but how to do so is uncertain. The two surface forcing processes, a result of turbulent atmospheric flow over waves, are boundary conditions for momentum transfer into the water column. The surface energy flux related to form drag is that which drives surface waves. As in other fluid dynamic flows where viscous and/or pressure stresses are involved, one supposes there exists a relevant Reynolds number. In this paper, we begin with the rather definitive near-wall data of Nikuradse, governed by a wall Reynolds number, and progress to equations useful for air flow over ocean surface waves for which reference is made to a consensus of formulas, resident in the literature, for drag coefficient versus wind speed. The processes of momentum transfer and the transfer of heat and water vapor across the air-sea interface differ. The governing equations of the latter scalar quantities do not contain pressure, and therefore, form drag is excluded. A detailed study of bulk coefficients for heat and water vapor transfer which includes low wind speed show that the bulk coefficients are nearly constants as a function of wind speed in accordance with observations. For greatest precision, neutral bulk coefficients presented here should be corrected for density stratification (Large and Pond 1982).
KeywordsAir-sea interaction Surface stress Ocean modeling
Discussions with Mark Donelan relevant to this paper are appreciated. Mark died in March 2018. He will be greatly missed.
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